1. Field of the Invention
The invention relates to a brake force control apparatus for a vehicle that performs front-rear wheel distribution control of brake force by individually controlling the brake pressure of the left and right rear wheels so as to obtain a target relationship between the slip degree index value of the left and right rear wheels and the slip degree index value of the front wheels during braking. More particularly, the invention relates to a brake force control apparatus and a brake force control method that perform front-rear wheel distribution control of brake force in a vehicle having a brake device provided with a brake pressure control system for the left front wheel and right rear wheel and a brake pressure control system for the right front wheel and left rear wheel.
2. Description of Related Art
A brake force control apparatus that performs front-rear wheel distribution control of brake force is available. For example, the below-described Japanese Patent Application Publication No. 8-301092 (JP-A-8-301092) describes a brake force control apparatus that performs front-rear wheel distribution control of brake force by controlling the brake force of the rear wheels so that the wheel speed of the rear wheels does not exceed the wheel speed of the front wheels during braking. In particular, in the brake force control apparatus described in JP-A-8-301092, the brake device of the front wheels is determined to have failed and the front-rear wheel distribution control of brake force is prohibited when the deceleration of the vehicle during braking is less than a predetermined value or when the difference in deceleration between the front and rear wheels does not have a maximum value (local maximum value).
Deceleration of vehicle and deceleration of wheels is affected by changes in loading conditions of the vehicle and changes in road surface gradient. Therefore, when failure determination of a brake device is based on deceleration of vehicle or deceleration of wheels, the failed brake device is sometimes determined to be normal and the front-rear wheel distribution control of brake force is sometimes implemented inappropriately. Conversely, a normal brake device is sometimes determined to have failed and the necessary front-rear wheel distribution control of brake force is not performed.
In particular, special attention should be paid to the front-rear wheel distribution control of brake force in the event of a failure when the brake device is a hydraulic brake device of a diagonal two-system (the so-called X piping) type, that is, a brake device provided with a brake force control system for the left front wheel and right rear wheel and a brake force control system for the right front wheel and left rear wheel.
For example, let us consider a case in which a failure has occurred in a brake force control system for the left front wheel and right rear wheel and the brake pressure of the left front wheel and right rear wheel has not assumed a necessary value. In such a case since the brake force of the right front wheel and left rear wheel is controlled normally, the wheel speed of the right front wheel is lower than the wheel speed of the left front wheel, and the brake pressure of the left and right rear wheels is individually controlled by reduction so as to obtain the predetermined relationship between the wheel speed of the left and right rear wheels and the wheel speed of the left front wheel. Since the brake pressure of the right rear wheel is inherently a low pressure due to the failure, the brake pressure of the left rear wheel is reduced comparatively significantly by the reduction control and the brake force of the left rear wheels is also reduced.
As a result, the difference between the sum of brake forces of the right front and rear wheels and the sum of brake forces of the left front and rear wheels increases, the excess yaw momentum in the right turn direction caused by the difference in brake force between the left and right wheels increases and travel stability of the vehicle decreases. Further, since the brake force of the entire vehicle decreases, the deceleration of the vehicle is less than the deceleration desired for the driver.
Therefore, in the front-rear wheel distribution control of brake force of a vehicle having a hydraulic brake device of a diagonal two-system type, whether or not the brake device has failed should be accurately determined and the increase in excess yaw momentum and decrease in vehicle deceleration should be prevented even when either of the systems has failed. In a vehicle having a hydraulic brake device of a diagonal two-system type, the abovementioned problems are encountered when the front-rear wheel distribution control of brake force is performed by individually controlling the brake pressure of the left and right rear wheels.